1. Field of the Invention
This invention relates to a process for producing a sheet having a beautiful chip-like pattern which is widely used as a floor material or the like.
2. Discussion of the Prior Art
According to conventional techniques, sheets of this nature are generally produced by a method which comprises spreading chips onto a matrix sheet and pressing the sheet under heat using a press or rolls. When a press is used the sheets are difficult to continuously produce and the efficiency of production is reduced. When rolls are used continuous production is possible but clear and beautiful products of high market value are difficult to obtain because the chips tend to partially overlap one another, or the chips are greatly deformed resulting in a chip-pattern having a vague contour, or drifting of the chip pattern occurs in the extrusion direction of the rolls.
In an attempt to overcome these disadvantages, a method for producing a chip-embedded sheet has recently been reported which comprises continuously feeding chips onto the surface of a matrix sheet using a microvibrating chip feeding plate in such a manner that the chips do not overlap one another, and consolidating the chips and sheet under heat and pressure to embed the chips in the matrix sheet (see Japanese patent application (OPI) No. 112873/76). (The term "OPI" as used herein refers to a "published unexamined Japanese patent application".) This method has been successful to some extent in solving the aforesaid problems, but there are still some drawbacks to be overcome.
One problem which occurs when embedding chips in the matrix sheet in the above manner is that a sheet in which the chips penetrate all the way through the sheet in its thickness direction cannot be obtained even when the embedding pressure is considerably high. Whether the chips penetrate through the sheet in its thickness direction greatly affects the service life of the sheet used as a floor material. For example, in a sheet obtained by the above method in which the chips are embedded halfway, when the sheet is worn down to the embedded depth, the chip pattern disappears. Thus, as compared with a sheet in which the chips penetrate from one surface of the sheet to the other whose chip-pattern does not disappear, the service life of the sheet having a beautiful chip-like pattern is shortened considerably. This difference is particularly noticeable when local wear tends to occur on the sheet.
Another problem with the aforementioned method is that considerably rigorous control must be exercised over various conditions during manufacture including the amount of chips fed, the amount of the matrix sheet fed, and the chip embedding pressure. For example, when the balance between the amounts of the chips and the matrix sheet at the contact point of press rolls is lost and the amount of the matrix sheet becomes excessive, the spaces between the chips which have been forced into the sheet increases and the resulting sheet having a chip-like pattern has a low chip filling density. On the other hand, when the amount of the chips becomes excessive, the chips overlap and the chips are compressed by a high embedding pressure and the contours of the chips become vague. Thus, in order to obtain products of uniform high quality continuously, very rigorous control is necessary. In practice, it is very difficult to constantly exercise this control and some variations in product must be accepted.
Still another problem is that it is difficult to embed chips in a compact dense state. Since the chips slide as they are fed onto the matrix sheet by means of a micro-vibrating chip feeding plate (set at an inclination angle of 5.degree.), the chips can be embedded in compact state to some extent when the feed rate (velocity) of the matrix sheet is low. However, as the feed rate of the matrix sheet increases, the feeding of the chips tends to be retarded and considerable spaces appear in the chip feeding direction. Accordingly, when it is desired to obtain a sheet having chips somewhat densely embedded, it must naturally be done at the sacrifice of a reduction in the efficiency of production.
The purpose of the present invention is to solve all of the aforesaid problems, and to provide a process for continuously producing with good efficiency a sheet having a chip-like pattern in which the chips are densely aligned in a plane without overlapping and substantially penetrate through the sheet in its thickness direction, and in this state, they are bonded integrally in sheet form by a binder, such that the chips have a clear contour and the sheet has a long service life.